With a recent trend toward a high density and miniaturation of semiconductor devices, plasma processing apparatus have been used to perform such processes as film forming, etching, ashing, and the like in a fabrication process of the semiconductor devices. Specifically, in a microwave plasma processing apparatus for producing a plasma using a microwave, the plasma can be produced stably under a relatively low pressure (high vacuum) condition of about 0.1˜10 Pa. Thus, the microwave plasma processing apparatus using, e.g., a microwave having a frequency of 2.45 GHz has attracted considerable attention.
An example of such a conventional plasma processing apparatus will now be explained. As shown in FIG. 6, the plasma processing apparatus includes a chamber 101 accommodating therein a substrate 111 and performing a predetermined process thereon; a high frequency power supply 105 for generating a microwave; and an antenna unit 103 for irradiating the microwave into the chamber 101.
The antenna unit 103 is formed of a slot plate 103c, a wave retardation plate 103b, and an antenna cover 103a. In the slot plate 103c, there are provided a plurality of slots (openings) for irradiating the microwave towards the inside of the chamber 101. The microwave generated from the high frequency power supply 105 is provided to the antenna unit 103 by a waveguide 106.
A top plate 104 serving as a part of a partition wall of the chamber 101 is disposed in an upper part of the chamber 101. The top plate 104 is made of a dielectric material, e.g., a quartz. A sealing member 114, e.g., an O-ring or the like, is provided between the top plate 104 and the partition wall of the chamber 101. The antenna unit 103 is disposed above the top plate 104.
In the chamber 101, a susceptor 107 for supporting the substrate 111 accommodated therein is provided. Further, a vacuum pump 109 is connected to the chamber 101 in order to vacuum-exhaust the inside thereof.
In the above-described plasma apparatus, the inside of the chamber 101 is exhausted to vacuum by the vacuum pump 109, and e.g., an argon gas is introduced into the chamber 101 as a gas for producing a plasma under a predetermined pressure range.
As shown in FIG. 7, the microwave generated by the high frequency power supply 105 arrives at the antenna unit 103 via the waveguide 106. The microwave arriving at the antenna unit 103 propagates through the wave retardation plate 103b, as indicated by arrows, and is irradiated into the chamber 101 via the slot plate 103c to thereby generate an electromagnetic field therein.
The argon gas is dissociated by the electromagnetic field generated in the chamber 101 to thereby forming a plasma generation region 122 between the substrate 111 and the top plate 104, and thus a predetermined plasma processing is performed on the substrate 111.
In the plasma generation region 122 formed in the chamber 101, electrons and ions (charged particles) present in the plasma generation region 122 oscillate with predetermined plasma frequencies to maintain the plasma generation region 122 in an electrically neutral state. The plasma frequency tends to increase as a charge density is high and a mass of the charged particle is small.
Therefore, in the plasma generation region 122, the plasma frequency of the electron having a mass substantially smaller than that of the ion is considerably high compared to that of the ion, and it is in a microwave region. At this time, if the frequency of the microwave generated by the high frequency power supply 105 is higher than the plasma frequency, the microwave can propagate through the plasma generation region 122 and be supplied into the plasma generation region 122.
Meanwhile, the plasma frequency of the electron is heightened as the electron density becomes high. If the plasma frequency of the electron exceeds the frequency of the microwave generated by the high frequency power supply 105, i.e., if a cutoff frequency in the plasma generation region 122 becomes higher than the frequency of the microwave, such a phenomenon is observed that an electric field of the microwave is cut off at a surface of the plasma generation region 122. Namely, the microwave is reflected by the plasma generation region 122. This phenomenon is more strongly observed as the electron density becomes higher.
Thus, the plasma density cannot be higher even though the power of the microwave is increased, so that the plasma density becomes saturated in the plasma generation region 122.
Meanwhile, the top plate 104 needs to have a certain thickness to secure a strength of the chamber 101, whose inside is depressurized and to thereby sustain the atmospheric pressure. Generally, uncontrollable standing waves 121 of the microwave are formed in the top plate 104 with such a thickness, as shown in FIG. 7. Due to the formation of such uncontrollable standing waves 121, the uniformity of the plasma density distribution becomes deteriorated in the plasma generation region 122.
As described above, since the plasma density cannot be further increased in the plasma generation region 122 and the uniformity of the plasma density distribution cannot be further improved beyond a certain level, it is difficult to perform an efficient and uniform plasma processing on the substrate 111.